1. Field of the Invention
This invention pertains to hoisting cranes, and more particularly, to safety features of hoisting cranes to prevent dropping or damaging the load because of a failure within the hoist system, and to protect the entire system from the forces resulting from load hang-up, or two-blocking.
2. Description of the Prior Art
Many cranes, such as nuclear fuel-handling cranes, require extreme failure-proofing safety measures because the potential consequences of dropping a load, due to failure of one of the components of the crane, may be disastrous. Two occurrences which can lead to failures, result when: (1) the traveling block of the crane reeving system accidentally engages the stationary or head block of the reeving system (known as two-blocking), or (2) the load or traveling block catches or hangs-up on some structural obstruction as the load is being hoisted (known as load hang-up). When either of these situations occurs, the crane and the reeving components become in effect a rigid system. The kinetic energy of the high-speed rotating components and the energy input of the drive motor must then be dissipated by elastic and/or inelastic deformation of the weakest member of the system--frequently leading to its failure. Often the cables are the weakest component of the crane and many times they fail--allowing the load to drop uncontrolled. Even if failure does not occur, the stresses to which the hoist and other crane components have been subjected cannot be determined and thus the remaining factor of safety of the crane is suspect. Additionally, the load itself must be protected from being torn apart during a load hang-up. The forces required to damage the load will vary with its strength, but are generally less than those which can be exerted by the crane's machinery.
In the past, various redundant switches have been placed in the vicinity of the crane reeving system so that as the traveling block approaches the stationary block the switches will de-energize the motor to bring the crane to rest. However, in some cases these switches fail due to improper installation, maintenance or wear. Secondly, these switches do not provide a safeguard against load hang-up. A second technique has been to provide substantial crushable or sacrificial structure between the traveling block and the stationary block. The purpose of this structure is to absorb the kinetic energy of the crane components so that it will be dissipated prior to the formation of a rigid system. Whether used with or without the switches, this technique is extremely costly and still does not prevent over-stressing and failure in the event of a load hangup or overload, during which the motor breakdown torque may be applied.
Clutches have been utilized in construction-crane drive trains to protect against overloads, but have not been utilized in overhead cranes because the clutch, or energy-absorbing torque-limiting device, would make the crane susceptible to loss of control of the load in the event of a mechanical failure of the clutch--an unacceptable risk for cranes used in critical service.
Drum emergency holding devices have been used to hold cable drums in a stationary position, but have not been used to suddenly stop hoists' drive systems, since the impact loading of the sudden stop on the drive train cannot be accurately evaluated. Furthermore, the large amount of rotational kinetic energy of the high-speed hoisting equipment would probably be absorbed in inelastic deformation of the drive train which could lead to failure or subsequent maloperation of the equipment.
Duplicate drive trains have been relied upon in the past to protect critical loads in the event of a single mechanical failure of a drive train component. However, merely duplicating the drive train components penalizes the hoist design in terms of the cost and weight of the extra equipment. Furthermore, the substantial increase in weight of the additional equipment that must be supported by the girders contributes to even more cost. Also, duplicate drive trains are still subject to common-mode failures and thus they do not provide diverse redundancy.